Foreign article on the way about concert is inflexible (14)
I believe the answer is:
uncompromising
'inflexible' is the definition.
('uncompromising' can be a synonym of 'inflexible')
'foreign article on the way about concert is' is the wordplay.
'foreign article' becomes 'un'.
'on the way' becomes 'coming' ('coming' can be a synonym of 'on the way').
'about' is an insertion indicator.
'concert' becomes 'prom' (abbreviation for promenade concert).
'prom'+'is'='promis'
'coming' enclosing 'promis' is 'compromising'.
'un'+'compromising'='UNCOMPROMISING'
(Other definitions for uncompromising that I've seen before include "firm" , "Not willing to make concessions" , "Obstinate" .)